Generally, a package of this kind for holding an integrated circuit (IC) chip has a lead frame provided with a chip mount on which the IC chip is mounted. The IC chip mounted on the chip mount is connected to the lead frame by bonding wire, and the IC chip and part of the lead frame are sealed in a resin, such as an epoxy resin, by transfer molding.
The package of this kind has been widely used, because the package of this kind uses a lead frame which is part of a plurality of successive lead frames in order to make an automatic holding process possible, and enable a batch type molding process.
Although the package of this kind can be mass-produced at a comparatively low cost, the package entails the following problems.
(1) Since a pitch which can be provided between adjacent edges of the inner leads of the lead frame by presently available techniques is on the order of 0.2 mm at the smallest, an increase in the circuit density, that is, an increase in the number of pin contacts, is limited by the pitch of the leads.
(2) Since a further reduction of the pitch of the inner leads of the lead frame is difficult, a further reduction of the intervals between bonding pads is difficult, and hence a large size package must be used for high-density holding requiring 200 to 500 pin contacts.
A package for holding an IC chip is proposed in Japanese Patent Laid-open (Kokai) No. 4-124867 to cope with such problems. A multilayer printed-circuit board serving as a relay board is disposed between a lead of the frame and an IC chip, the IC chip being connected electrically to the inner leads of the printed-circuit board by bonding wire and the outer leads of the printed-circuit board being connected electrically to the leads of the lead frame by bonding wire to thereby connect the IC chip electrically to the leads of the lead frame, and the printed-circuit board, the IC chip part of the leads of the lead frame are sealed in a resin molding by a molding technique.
The multilayer printed-circuit board is fabricated by laminating paper impregnated with epoxy resin, laminating paper impregnated with phenol resin, or fiberglass sheets impregnated with epoxy resins. The above relay board entails the following problems.
(1) The strength of the multi-layer printed-circuit board serving as a relay board is not sufficiently high because the multilayer printed-circuit board is made by laminating paper sheets or fiberglass sheets impregnated with a synthetic resin, such as an epoxy resin.
(2) There is the possibility that the relay board having paper sheets or fiberglass sheets impregnated with a synthetic resin can be distorted by the molding process. The relay board must have a comparatively large thickness to prevent being distorted, and thus a large size of the package is required.
(3) The thermal resistance of the relay board is not sufficiently high.
(4) The wiring patterns require complicated manufacturing processes, including etching processes for copper films or the like.